Controlling rear fill speakers

I am using one 4 channel amp to bi-wire and control front components and a two channel amp bridged to control the subwoofer.  I have an Alpine 9833 headunit running in 3 way mode to control.  I am using the headunit power to drive the rear fill Boston Acoustics NX57 5 3/4 speakers.  The problem is I have no control over the rear fill and when the volume gets loud, they distort and sound badly before the rest of the system.  What type of devices are there that I could use to passively control either frequency or output of these speakers?

Yea the whole HU Power/Amp power thing never really works that well.  It can, however, be done, and be done well.

There isn't anything you can do about HU power.. I've got the same deck and I *know* that its totally useless past about volume 20 or so on the deck. 

Your only opition is judicious use of the gain and fader controls.

I don't know what you mean by "3-way mode to control" but here is how you should have your stuff wired:

Front Stage RCA Outputs -->  Amplifier

Rear Stage HU Power Outputs --> NX Series coaxials

Sub RCA Outputs --> Subwoofer Amplifier

Now, FADE your deck forward.. alot.  Like at least to 7 or 8 or so.  This will reduce the amount of power sent to your rear speakers and help raise where on the volume knob they will start to distort..  I'm not sure by how much tho..you'll have to set this carefully.   Next, when you have a volume where your rears are not distorting and you still get a good signal to your front stage RCA's (On that deck I'd say a good 25 - 30... ) hook up your front speakers (or turn on the gain, or whatever) and get them to a spot where they sound good and loud.. but make sure you drive around a bit and rev the engine up.. when you start raising your gains higher than they otherwise should be (or raise them at all with crappy line voltage or wires) then you start picking up alternator whine and other nastiness...

It can be a real challenge to do this right, and depending on you're personal tastes you make not get it to sound properly at all... a small rear-stage amp *is* the best solution for absolute fidelity, but if you play around a bit I betcha, with that HU, you'll be able to get it sound right.  Don't be afraid to turn your front stage up *alot* though.. it'll really have to play catch up with that volume knob ...

Anywho, best of luck.